Line imagers can be operated with relative motion between the "platform" bearing them, and a field of view, to generate video signal samples that raster scan the field of view. A line imager typically comprises a line of photosensors for collecting photocharge and means for periodically carrying away the charge packets collected, then assembling them into serial format for sensing by a charge sensing circuit. The charge sensing circuit can be a floating-diffusion electrometer, for example; and the means for carrying charge packets to the electrometer can be charge-coupled device (CCD) circuitry. Line imagers may be operated to sense infrared or visible-light wavelengths.
Schottky barrier diodes formed from a metal silicide contact to silicon are favored photosensors for infrared wavelengths. This is because these photosensors can be fabricated directly on the silicon die alongside the CCD circuitry required for charge transport, with only a few additional processing steps besides those processing steps required to fabricate the CCD circuitry and the charge sensing circuitry that follows.
Photosensing for visible light, or near infrared, or both can be done with a silicon pn junction or in a potential energy well electrostatically induced in a portion of the surface of the silicon substrate apart from the CCD circuitry. When a visible-light-responsive solid-state imager is illuminated through its CCD-gate electrode-bearing surface, it is particularly desirable to do photosensing in a portion of the imager apart from the CCD circuitry. This is because the gate electrodes will interfere with obtaining proper response in the blue wavelengths, despite their being made of phototransmissive polysilicon.
Collecting photocharge for only one line scan interval in a line imager tends to provide inadequate photosensor response unless the levels of photosensor irradiation (in the wavelengths the photosensors respond to) are relatively high. In this regard line imagers tend to be less satisfactory than area imagers, where photocharge collection times tend to be longer. On the other hand, the shorter time for photocharge collection tends to make the line imager better in responding to moving objects without blurring effects. Electronic cameras which use line sensors to scan a surface of revolution in a pushbroom operation are a natural choice for aerial reconnaissance and for observations of the earth from a satellite; the line sensors are less complex to make and operate than area sensors. Where photoconversion takes place in photosensors separate from the CCD charge transfer circuitry of the sensor, better fill factors can be obtained using the line sensor.
To obtain freedom from blurring on certain types of relative motion between the imager platform and the position of the image to which the imager generates photoresponse, but to still obtain longer photocharge collection times for improved sensitivity, a mode of imager operation referred to as "time delay integration" or "TDI" is often resorted to. An area array of photosensors is employed, which photosensors are arranged in a number m rows and a number n columns. CCD charge transfer channels are interleaved with the columns of photosensors to be used as interline transfer registers, each channel having a succession of m charge transfer stages therein, which charge transfer stages may be considered to be consecutively ordinally numbered first throu m.sup.th in the direction of forward charge transfer. Photocharge is collected over line scan intervals and the charge packets are transferred in short register-loading intervals between line scan intervals, from each of first through m.sup.th rows of photosensors to a respective set of charge transfer stages of like ordinal number. This transfer is made after the previous charge packet contents of the charge transfer channels have been advanced by one charge transfer stage. The line of charge packets transferred in parallel from the output ports of the interline charge transfer channels during this one stage advance side-load the successive charge stages of an output CCD shift register, the forward clocking of which is suspended during register-load intervals. During line scan intervals the output CCD shift register is forward clocked to transfer the charge packets serially to a charge sensing stage. Inasmuch as the image elements move across the photosensor array in the direction its columns are oriented, at the same speed that the charge packets are transferred in the interleaved charge transfer channels, image integration time is lengthened. This is an auto-correlation process that improves the sensitivity of the imager for those elements. Other elements of the image are spatially low-pass filtered by time delay integration in the direction perpendicular to line scan. That is, they are blurred due to relative motion between them and the charge transfer process in the interline charge transfer channels.
(It is convenient to refer to the line scan interval and register-load interval as "line trace interval" and "line retrace interval", respectively, using the terms commonly used with regard to the kinescope displaying pictures generated from the video signal response of the CCD imager. This convention will be used throughout the rest of this disclosure.)
The insertion of interline charge transfer channels between the columns of photosensors in the photosensor array introduces larger non-photosensitive areas into the imaging area, creating a less acceptable spatial alias and lowering fill factor. "Fill factor" is the percentage of the imaging area from which photocharge can actually be collected and directly effects the photoefficiency of the imager. The interline charge transfer channels undersirably reduce fill factor to 35% in typical prior-art line transfer CCD imagers. The loss of sensitivity in the imager due to poor fill factor is readily recovered by time delay integration, but the aliasing problem is not overcome. Further, in some imager applications it is desirable to be able to select between time delay integration and staring modes of operation, and increased sensitivity may be desired for the staring mode of operation.
A relatively new type of staring area imager is the "charge-sweep-device" or "CSD" imager. Such an imager has been described by M. Kimata et al in a paper entitled "A 480.times.400 Element Image Sensor With a Charge Sweep Device" appearing in pp. 100, 101 of the DIGEST OF TECHNICAL PAPERS, 1985 IEEE INTERNATIONAL SOLID-STATE CIRCUITS CONFERENCE. The interline charge transfer channels are narrowed a few times in a CSD imager, allowing fill factors of 70%, for example. Charge packets are transferred a row at a time to the interline charge transfer channels, a successive one of the rows being transferred during each line retrace interval to be clocked forward at pixel scan rate during line trace interval to accumulate under a charge storage gate crossing the ends of the interline charge transfer channel. Individual charge packet transfers need not be complete at the pixel scan rate, so the interline charge transfer channels can be narrowed at the expense of efficient charge transfer, to allow closer packing of the columns of photosensors and thus increase the fill factor. During each line retrace interval, the line of charge packets accumulated under the storage gate is side-loaded into the output CCD shift register, to be clocked forward serially to the charge sensing circuit the succeeding line trace interval. The side-loading is carried out by lowering a potential energy barrier induced in the ends of the interline charge transfer channels, by changing the voltage on an overlying storage control gate interposed between the charge storage gate and the output CCD shift register, and then changing the voltage on the storage gate to reduce the depths of the potential energy wells induced thereunder.
The CSD imager, despite its attractiveness in reducing the non-photosensitive portion of the imaging area, cannot be operated in the time-delay-integration mode to increase its sensitivity still further. This is because only one line of charge packets can be transferred into and through the interline charge transfer channels during any time. A formidable technical problem is presented, then, by the desire to combine the advantages of CSD imaging and of TDI imaging.
The present inventor proposes to solve this problem with the CSD imager by using a new type of storage register inserted between the interline charge transfer channels and the output CCD shift register. This new type of storage register has the capability of accumulating successive charge packets supplied to it, which capability can be utilized to perform time-delay-integration.
This new type of storage register can also be used to perform other useful image processing functions. For example, this new type of storage register can be used as a temporary frame storage register to provide true line interlace between alternate field scans in a frame-transfer type of imager. Prior art frame-transfer imagers are only capable of providing psuedo line interlace from field to field.